Lightweight Aggregate Concrete

 Chapter no. 2

The use of LWAC can be traced back to ancient times. In Europe, using this form of concrete, Romans built the famous Pantheon about two thousand years ago. Even earlier, in Asia, in 3000 BC, the famous Mohenjo-Daro and Harappa were built during the Indus valley civilization again using a form of LWAC. Therefore, LWAC is not a new invention in concrete technology as it has been known since ancient times. During early ages, LWAC was made using natural aggregates of volcanic origin such as pumice, scoria, etc. Sumerians used this in buildings in Babylon in 3^rd millennium B.C (refer figure 2.1). The Greeks and the Romans used pumice in building construction. Some of these magnificent ancient structures still exist like; St. Sofia Cathedral or Hagia Sofia (shown in figure 2.2), in Istanbul, Turkey; the Roman temple Pantheon (refer figure 2.3) was erected in the years A.D.118 to 128; the prestigious aqueduct, Pont du Gard, built ca. A.D. 14 (shown in figure 2.4); and the great Roman Amphitheatre, Colosseum (refer figure 2.5), built between A.D. 70 and 82. In addition to building constructions, the Romans used natural LWA and hollow clay vases to reduce the weight of various members of structures. This technique was also used in the construction of pyramids (shown in figure 2.6) during the Mayan period in Mexico. [5]

LWA are the basic ingredient for making LWAC. In earlier times LWAs were of natural origin, mostly volcanic: Pumice, scoria, and tuff, etc, which were used both for fine and coarse aggregates. These aggregate are active and react with hydration products of cement forming excellent bond and imparting strength. With the increasing demand and the non-availability of natural LWAs worldwide, techniques have been developed to produce LWAs in factories. These are produced from the natural raw materials like expended clay, shale, slate, etc., as well as from industrial by-products such as fly ash, bed ash, blast furnace slag, etc. the properties of aggregates depend upon the raw materials and the process used for processing them.

         

                                                    Figure.2.1: View of Babylon built in the 3^rd millennium B.C.

         

                        Figure.2.2: View of St. Sofia Cathedral, Hagia Sofia, in Istanbul, Turkey.

Figure.2.3: View of the Roman temple, Pantheon, built in A.D. 118.

            Figure.2.4: View of the prestigious aqueduct, Pont du Gard, built in A.D. 14.

                                Figure.2.5: View of the Great Roman Amphitheatre, Colosseum, built between A.D.70 and 82.

                       Fig.2.6: View of Pyramids in Mexico, built during the Mayan Period, A.D. 624-987.

In the early days, porous Clay brick were also made up of naturally existing lightweight aggregate. These bricks were produced long before the Christian era, during Indus Valley civilization in 2500 B.C (refer figure 2.7). These were used in the construction of two cities, Mohenjo-Daro and Harappa. It has been found that these porous bricks were crushed and used as the lightweight aggregates in the masonry. Although the origin of the LWAC is difficult to assess, it would not be an exaggeration to say that its roots are from the ancient period.

                                                Figure 2.7: View of Mohenjo-Daro and Harappa, 2500 B.C.

With the increase in the demand of LWAC and the unavailability of the aggregates, technology for producing lightweight aggregate has been developed. In Germany, in the 19^th century, porous clay pieces were produced by quick evaporation of water. The industrial use of natural lightweight aggregates in Germany was started in 1845 by Ferdinand Nebel from Koblenz who produced masonry blocks from pumice, using burnt lime as the binder [5].

Today, LWA are produced in every wide range of densities varying from 50 kg/ for expanded perlite to 1000 kg/ for clinkers. With these aggregates and light range water reducers, it is possible to make LWAC of 80 MPa cube compressive strength.

Because of the practical advantages which it possesses, LWAC has, in recent years, become an important structural material and the demand for it is increasing. A saving in the weight of superstructure means that the foundations can be reduced in bulk, and time and expenses saved in erection and handling of components, so that smaller lifting equipment’s can be employed or larger precast units can be handled.

The low density results in high thermal insulation of buildings and, in some instances, the thickness of roofs and walls can be reduced. Where there is reduction in weight, a higher degree of thermal insulation will be achieved.

Nearly all LWACs are inherently fire resistant. In addition, depending upon density and strength, the concrete can be easily cut, nailed, drilled, and chased with ordinary wood-working tools. One simple example in 4L concrete developed at the Chalmers University of Technology, Goteborg, Sweden. The name is given on the basis of three properties: LWAC, low density and strength lesser than 20 MPa.

It is generally understood that concrete is not necessarily just heavy, sharp- edged grey blocks. It can acquire any shape color, density, and strength. The low density of pumice aggregates results in weight reduction of the structures and the foundations, and provides considerable savings regarding thermal insulation. The low density of the material results in high thermal insulation for buildings and in some instances, the thickness of roofs and walls can be reduced. Where there is no reduction in thickness, a higher degree of thermal insulation will be achieved. The density, example range from 300 to 3000 kg/m³; thermal conductivity from 0.1 to 3 W/mK; and strength from 1 to 100 MPa or even more. Figure 2.8 shows the type of LWA and their corresponding density in lb/ft³. The density is mostly controlled by the type of aggregate used. The strength is also partially dependent upon the type of aggregates used for making the concrete.

                                                Figure 2.8:Various types of LWA and range of their densities

 

The natural aggregates, Pumice and Scoria, for example. These can be made into concrete weighing about 50 pounds, and it also may run as high as 65 pounds per cubic foot.

Overlapping these are coal cinders, with a range from 75 to 120 pounds, and expanded shale, clay and slate aggregates produced by the rotary kiln method, which will produce a structural concrete ranging from 85 to 115 pounds per cubic foot. Expanded shale, clay or slate produced by sintering, and expanded slag, range from 90 to 120 pounds per cubic foot and complete and complete the spectrum.

Beyond this, there are the air-cooled slag aggregates and the hard-rock aggregates such as sand and gravel and crushed stone, which produce conventional concretes weighing 135 to 150 pounds per cubic foot.

Locally, in the city of Peshawar a pilot plant for production of the LWA from slate has been set. The Complex has already about supplied 850 tons of the material for use in the Khairabad bridge project and about and 80 tons for the construction of Sherpow bridge in Lahore. Besides above, about 11 tons of LWA was supplied for Council for Works and Housing Research (CWHR) Karachi, and 22 tons for National Physical and Standard Laboratories (NPSL) Islamabad [6].

In 1918, Stephen J. patented the LWA “Haydite” the first one made by the expansion of shale, which came into production in the US. Synthetic aggregates of this type also have been universely accepted, making satisfactory reinforced and prestressed concrete.

The first building frame of reinforced LWAC in Great Britain was a three story office block at Bent ford, near London, built in 1958, since then, many structures have been built of precast, in-situ prestressed, or reinforced LWAC.

In the UK, clinker aggregate concrete was used in the construction of the British museum in the early part of 20^th century. The output of the clinker aggregates increased enormously in the ensuing decades, but its manufacture is now declining since oil and other fuels are more widely used for firing the furnaces. This trend is likely to continue and, as result, the use of sintered pulverized fuel ash (fly ash) has been steadily increasing over the last decades. Besides this, other types of ashes have also been used for producing aggregates like bed ashes from boilers, etc.

With an increase in off-shore construction, and the general reluctance of using LWAC due to its low density and strength, the demand for improved strength has increased. This has led to the development of high strength structural LWAC, specifically in Norway, the low strength of aggregates has been balanced by using high strength cement mortar, because the high strength mix has a dense pore structure, it may increase the insulating properties in comparison to the normal  strength LWAC.